382 STRUCTURE OF THE EYE. 



were, to take in almost a hemisphere at a glance, this extension of the 

 visual function can only be accomplished in one of two different ways. 

 rent con ^ use ^ e illustration we have been employing, it may be 

 trivances for reached by having innumerable cameras pointing in innumer- 

 iSger field of a ^ e directions, and conveying the resulting images to one 

 view by the common surface, or by having one, or at most two, camerse 

 set upon a movable stand, which can quickly point them in 

 any direction, and so enable them to inspect successive fields of view 

 with almost instantaneous rapidity. The former plan is resorted to in 

 most insects, the latter in man. In insects, the immobility of the head 

 upon the trunk would interfere with any rapid rotation of the visual or- 

 gan ; in man, the facility with which rotation can take place upon the 

 neck as on an axis, and the movement of the eye in its orbit, accom- 

 plishes the object without any kind of difficulty. 



In continuing an investigation of the structure of the eye, it is con- 

 venient to consider it under three heads : 1st. Its optical mechanism ; 

 2d. Its nervous mechanism ; 3d. Its accessory apparatus. 



1st. Of the Optical Mechanism of the Eye. 



The human eye is of a globular form, and about one inch in diameter. 

 The human I* * s no ' t perfectly spherical, its lateral diameter being shorter 

 e 3 Te - than its antero-posterior by about one twentieth part. It may 



be described as consisting of three coats, which, forming a shell, contain 

 transparent media and the optical apparatus. It might also be consid- 

 ered as arising from an expansion of the optic nerve into an almost 

 spherical cavity, and which, being fortified by certain tissues behind, 

 has a dioptric mechanism in front. 



The coats of the eye are three in number : the sclerotic, the choroid, 

 and the retina. The sclerotic, which is the exterior, is a white fibrous 

 membrane, very tough, and possessing the necessary resistance to give 

 mechanical protection to the parts within. Within this is the choroid, a 

 vascular layer or tunic, presenting on its interior the black pigment which 

 darkens the interior of the eye. The innermost coat is the retina, an ex- 

 pansion of the optic nerve. The sclerotic coat is perforated in front, and 

 into the circular aperture so arising the transparent cornea is let, like a 

 watch-glass. . Many anatomists, however, consider that the cornea is 

 absolutely continuous with the sclerotic, and a part of it ; the sclerotic 

 and the choroid are united round the edge of the cornea by the ciliary 

 ligament. The iris is perforated in its centre, the aperture being desig- 

 nated as the pupil. Posterior to the iris is the crystalline lens, the space 

 between the lens and the cornea being filled with the aqueous humor, in 

 which the iris floats, dividing it into two regions, called, from their posi- 

 tion, the anterior and posterior chambers. All the rest of the ,globe be- 



